The present invention relates to transconductance cells, in particular, to linearized transconductance cells for use in electronic circuits.
A transconductance cell is an electronic building block used to build more complex electronic circuits. It is widely used in RF circuits such as low noise amplifiers and Gilbert cell mixers.
The transconductance cell performs the function of converting a voltage input into a current output. The characteristics of a desirable transconductance cell are high bandwidth, low noise, low power consumption, high output impedance, low distortion and good common mode rejection.
FIG. 1 shows a schematic diagram of a prior art transconductance cell 20. Other parts of the RF circuit (e.g., the Gilbert cell mixer) to which the transconductance cell 20 is connected are not shown. The prior art transconductance cell 20 for RF applications includes a first bipolar transistor 22, a second bipolar transistor 32, a first inductor 24, a second inductor 30, a resistor 26 and a capacitor 34. A voltage input 21 is coupled to the base of the first transistor 22. The collector of the first transistor 22 is coupled to another part of the circuit. The emitter of the first transistor 22 is coupled to one end of the first inductor 24. The opposing end of the first inductor 24 is coupled to one end of the second inductor 30. One end of the resistor 26 is coupled to a node between the first and second inductors 24 and 30. The other end of the resistor 26 is coupled to a node 28 which may be a ground.
The opposing end of the second inductor 30 is coupled to the emitter of the second transistor 32. The collector of the second transistor is coupled to another part of the circuit. The base of the second transistor 32 is coupled to the capacitor 34 and is biased at a constant voltage.
In operation, a bias voltage is applied to the transconductance cell 20 at the bases and the collectors of the first and second transistors 22 and 32 to bias the first and second transistors 22 and 32 for operation. In response to the bias voltage, DC currents flow through the first and second transistors 22 and 32 and exit through the resistor 26. The bias voltage typically ranges between 2.7 volts and 5.5 volts for RF circuits. Much of the bias voltage is dropped across the resistor 26 which is designed to have high impedance, as explained below. The resistor 26 also can be implemented as a transistor which also cause a voltage drop.
Once the bias voltage has been applied, the voltage input 21 is applied to the base of the first transistor 22 to output a signal from the emitter of the first transistor 22. The signal travels through the first and second inductors 24 and 30 and is output from the collector of the second transistor 32 to another part of the circuit. The signal output by the first transistor 22 may be directed from the first inductor 24 to the second inductor 30 without significant signal dissipation through the resistor 26 by using a resistor that has a high impedance value as the resistor 26.
One problem associated with the prior art transconductance cell 20 is that this requisite high impedance of the resistor 26 makes it difficult to operate the circuit at a low voltage.
Another problem associated the prior art transconductance cell 20 that the noise factor (NF) and the third-order input intercept point (IIP3) are degraded due to signal loss in the resistor 26. The noise contribution of the resistor 26 lowers the NF as well.